Field of the Invention
The invention relates to a monitoring apparatus for the sheet feed to a sheet-processing machine (e.g., a sheet-fed printing machine), in particular overlapping sheet feed. The monitoring apparatus has at least one sensing element which is disposed above or beneath the sheet stream and, by means of an actuating device having an actuating drive, can be displaced in the direction of the sheet stream and in the opposite direction. The invention further relates to a method of monitoring the sheet stream structure or the sheet stream during its transport into a sheet-processing machine, wherein at least one sensing element can be displaced in the direction of a back-pressure element, over which the overlapping sheet stream or the sheet stream having individual sheets is guided.
U.S. Pat. No. 4,753,433 (European patent EP 0 242 622 B1) describes a monitoring apparatus of the type mentioned here which has a sensing roller which is arranged above a feeder table and can be displaced in the direction of the feeder table with the aid of an electric stepping motor. Coupled to the sensing roller is a sensor which always transmits a signal to a control device when at least two sheets overlap each other. It has been shown that the monitoring apparatus can be used only to a limited extent and, in the cases wherein dynamic and very precise displacement of the sensing roller is required, cannot be used, among other things because of the relatively sluggish stepping motor.
The object of the present invention is to provide a monitoring apparatus which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and to provide a method of monitoring the sheet stream wherein high functional reliability can be ensured.
With the above and other objects in view there is provided, in accordance with the invention, a monitoring apparatus for the sheet feed to a sheet-processing machine wherein a stream of sheets travel in a given sheet stream direction, comprising:
at least one sensing element disposed adjacent (i.e., above and/or underneath) the sheet stream;
an actuating device connected to the sensing element and configured to selectively displace the sensing element towards and away from the sheet stream, the actuating device having an actuating drive for displacing the sensing element, the actuating drive including at least one piezoelectric actuator (i.e., it is has or is formed by the piezoelectric actuator).
In other words, the apparatus has one or more sensing elements which is/are arranged above or beneath a sheet stream and can be moved in the direction of the sheet stream and away from the sheet stream by means of an actuating drive. The monitoring apparatus is distinguished by the fact that the actuating drive has at least one piezoelectric actuator or is formed by the latter. With the aid of the piezoelectric actuator, dynamic, that is to say very fast and precise, displacement of the sensing element is possible, which may be necessary in specific monitoring operations.
In a particularly advantageous exemplary embodiment of the monitoring apparatus, the piezoelectric actuator is constructed as described in German patent DE 196 46 511 C1 of the company marco Systemanalyse und Entwicklung GmbH. The disclosure content with regard to the construction and the function of the piezoelectric actuator of the German patent is herewith incorporated by reference. The piezoelectric actuator has a basic element which has a stack structure built up from ceramic lamellae layers.
By means of appropriate electrical driving of the piezoelectric actuator the latter can be pivoted or tilted about an axis lying at the area center of gravity of the basic element, so that the sensing element, directly or with the interposition of a transmission mechanism, can be displaced in the direction of the sheet stream or, respectively, of a support over which the sheet stream is transported, and in the opposite direction.
Further advantageous exemplary embodiments of the monitoring apparatus emerge from the dependent claims.
With the above and other objects in view there is also provided, in accordance with the invention, a method of monitoring a sheet stream structure or a sheet stream being transferred into a sheet-processing machine, which comprises:
transporting a stream of sheets across a back-pressure element and towards a sheet-processing machine;
displacing at least one sensing element in a direction towards the back-pressure element over which the stream of sheets is guided in the form of an overlapping sheet stream or a sheet stream having individual sheets;
detecting, with the sensing element set against the sheet stream, the sheet stream as it is transferred into the sheet-processing machine; and
thereby displacing the sensing element with respect to the sheet stream in dependence on an instantaneous sheet stream thickness such that a pressing force with which the sensing element is pressed against the sheet stream is maintained within a predefined range.
In other words, in order to monitor the sheet stream structure or the sheet stream during its transport into a sheet-processing machine, at least one sensing element can be displaced in the direction of a back-pressure element, over which the overlapping sheet stream or the sheet stream having individual sheets is guided, that is to say transported. The method is distinguished by the fact that the sensing element is set against the sheet stream, that is to say pressed on with a defined force, and detects the sheet as it is transferred into the machine, for example a sheet-fed printing machine. During the monitoring of the sheet stream, the sensing element is always displaced with respect to the sheet stream as a function of the respective instantaneous sheet stream thickness in such a way that the pressing force with which the sensing element is pressed against the sheet stream is kept at a predefinable value or within a predefinable value range. By means of a suitable sensor system, the travel is determined by which the sensing element is displaced with respect to the preferably stationary back-pressure element, for example formed by a feeder table. In this way, the thickness of the sheet stream can be determined or measured very exactly, so that double and missing sheets can readily be detected, which, for example, leads to the output of a fault signal. In addition, by means of the change in the distance of the sensing element with respect to the back-pressure element, including the respective instantaneous machine speed, premature, delayed and skewed sheets can also be detected. The method is distinguished by high accuracy and, in addition, exhibits high functional reliability. Moreover, it is particularly advantageous that, because of the pressing force of the sensing element on the back-pressure element being kept constant, the measuring conditions are constant, irrespective of the respective sheet stream thickness. Automation of the sheet thickness measurement or the sheet stream thickness measurement and/or the sheet stream monitoring is therefore readily possible.
In an advantageous exemplary embodiment of the method, provision is made for the pressing force of the sensing element against the sheet stream to be monitored and, in the event of the pressing force falling above or below a limiting value, for a fault signal to be triggered which, for example, leads to a visual display of a fault or directly to the machine being stopped.
According to another design variant, the distance between the sensing element and the back-pressure element is monitored by the relative movement of the sensing element with respect to the back-pressure element being measured/monitored in order to set a constant pressing force, it being possible here, too, for a fault signal to be output as soon as the distance between the sensing element and the back-pressure element falls above a specific upper limiting value or below a specific lower limiting value. This means that if, for example, a double sheet occurs within the sheet stream, so that at this point the thickness of the sheet stream is impermissibly increased, the sensing element will be moved so far away from the back-pressure element, with the aid of a suitable, preferably dynamic, actuating drive, that the pressing pressure of the sensing element against the sheet stream remains at a constant value. In this case, however, if the sensing element is moved beyond a maximum permissible distance from the back-pressure element, a desirable message is provided in the form of a signal.
There is also provided, in accordance with the invention, a method of monitoring a sheet stream structure or a sheet stream being transferred into a sheet-processing machine, which comprises:
transporting a stream of sheets across a back-pressure element and towards a sheet-processing machine;
displacing at least one sensing element in a direction towards the back-pressure element over which the stream of sheets is guided in the form of an overlapping sheet stream or a sheet stream having individual sheets;
detecting a sheet stream structure with the sensing element in contact at a start of the sheet transfer;
determining a height profile of the sheet stream from information obtained in the sheet stream detecting step;
setting the sensing element away from the sheet stream and, during a further transfer of the sheet stream, continuously displacing the sensing element in the direction of the sheet stream and in the opposite direction such that a clear spacing between the sensing element and the sheet stream is substantially constant; and
if a fault in the sheet stream is detected, outputting a fault message.
In other words, in order to monitor the sheet stream structure or the sheet stream during its transfer into a sheet-processing machine, provision is made firstly for the structure of the overlapping sheet stream or the sheet stream having individual sheets to be detected, that is to say it is determined how the sheet stream thickness changes. To this end, in a preferred embodiment, at the start of the sheet transfer, firstly the sensing element is set against the back-pressure element, that is to say brought into contact and pressed on with a defined force, in order for example to set the zero point and to determine the machine oscillations. The sheet stream is then transported into the nip formed between the back-pressure element and the sensing element, so that the sensing element is then pressed against the sheet stream with a preferably defined, in particular constant, force. Detection of the sheet stream with contact by the sensing element therefore takes place. From the information determined by the sheet stream detection, with the aid of a suitable device, for example a control and regulating device, the contour height profile of the sheet stream is determined. The sheet stream height profile is determined by the different thickness of the sheet stream, which may be overlapping and or have individual sheets arranged at a distance from one another. After the height profile has been determined, the sensing element is set away from the sheet stream, that is to say spaced apart from the sheet stream, and during the further transfer of the sheet stream, is always displaced in the direction of the sheet stream and in the opposite direction, that is to say away from the sheet stream, in such a way that the clear spacing between the sensing element and the sheet stream is constant or substantially constant. The sensing element therefore travels over the sheet stream structure without contact, the displacement movement of the sensing element for setting the constant spacing of the sensing element from the sheet stream being carried out as a function of the machine speed.
The clear spacing between the sensing element and the sheet stream is so small that, if there is a double sheet in the sheet stream, contact occurs between the double sheet and sensing element. This xe2x80x9ccollisionxe2x80x9d can be determined in various ways, for example by determining the force exerted by the double sheet in the event of contact between the latter and the sensing element. After the double sheet has been detected, constituting a fault in the sheet stream, a fault message is output, which, for example, leads directly to the machine being stopped. Further faults in the sheet stream are premature and delayed sheets, which therefore arrive too early or too late at the sensing element, counter to the idealized sheet stream determined by the height profile, and are therefore detected as premature or delayed sheets on the basis of the time of their contact with the sensing element, which can readily be monitored by means of a suitable device. Using this method of monitoring the sheet stream, although no missing or skewed sheets can be detected, this method has the advantage that on account of the essentially non-contacting detection of the sheet streamxe2x80x94except at the beginning of the sheet stream transportxe2x80x94marking of the sheets can be ruled out. The method can therefore be particularly advantageously used in the case of very sensitive sheets.
In a preferred embodiment of the method, the sensing element is displaced in an oscillatory manner as a function of the machine speed, in order to monitor the sheet stream without contact for double or multiple sheets. To this end, the sensing element arranged above or below the sheet stream is moved linearly or along a circular path section.
In order to be able to ensure a constant spacing between the sensing element and the top of the sheet stream continually, one advantageous exemplary embodiment, wherein the sheet stream has individual sheets spaced apart from another, provides for the sensing element, in a first end position defined by the height profile of a fault-free sheet stream, to dip into every clearance which is formed between a preceding sheet and a sheet following immediately, that is to say it is moved in. If the sheet stream is not fault-free and, for example, has a premature, delayed or skewed sheet, the sensing element is pressed against the sheet stream, which can be determined by means of a sensor system, for example.
In another exemplary embodiment, wherein the sheet stream is overlapped, that is to say at least two sheets partially overlap one another, the sensing element, in a first end position defined by the height profile of the fault-free sheet stream, is moved into every depression in the top of the sheet stream, between the trailing edge of a preceding sheet and the trailing edge of a following sheet. In the case of a double or multiple sheet, the sensing element makes contact with the sheet stream, which leads to a fault indication.
It remains to be recorded that, in the case of this method according to the invention, there is always contact between the sensing element and the sheet stream only when the clear spacing, determined with the aid of the determined height profile of the sheet stream, between the sensing elements located in its first end position and the back-pressure element or the sheet stream is reduced impermissibly on account of a double/multiple sheet or the like.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a monitoring apparatus for the sheet feed to a sheet-processing machine, and method of monitoring the sheet stream structure/the sheet stream, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.